1. Technical Field
This invention relates to the art of depositing polycrystalline diamond films by low pressure chemical vapor deposition, and more particularly to hot filament chemical vapor deposition techniques.
2. Discussion of the Prior Art
The technology of low pressure chemical vapor deposition (hereinafter CVD) of polycrystalline diamond films has advanced in recent years (see J.C. Angus and C.C. Hayman, Science 241, 913 (1988); B.V. Spitsyn, L.L. Bouilov, and B.V. Derjaguing Journal of Crystal Growth 52, 219 (1981); and S. Matsumoto, Y. Sato, M. Kamo, and N. Setaka, Japanese Journal of Applied Physics 21, L183 (1982)). However, fundamental understanding of the chemistry and Physics occurring in this CVD process is still lagging. The mechanism by which diamond is formed is not clear to the prior art, particularly with respect to intermediate substances that-may be required for stimulating diamond crystal growth.
This invention is directed to a method of using hot filament CVD that makes more clear the role of intermediate substances. Hot filament CVD is desirable because of its (a) low capital cost, and (b) ease to scale up. It is important to distinguish other low Pressure CVD techniques from hot filament CVD; such other techniques may comprise activation by radio frequency (RF) or microwave plasma, ion electron assisted CVD, RF sputtering, and ion beam. These other techniques are substantially different because the formation of electrically charged species will proceed in different chemical and physical reactions than those of neutral radical species produced by hot filament CVD.
Although hot filament CVD is desirable from an economic and scale up capability standpoint, it is limited by filament durability and accepted feed gases to produce less than optimum quality diamond. This invention has discovered a way to deposit diamond at high growth rates and with a desirable quality using hot filament CVD but with hydrocarbon feed gases which do not contain methyl group gases and at lower equivalent filament temperatures.
Two bodies of prior art are pertinent to this discovery: hot filament CVD using methyl containing feed gases, and other low pressure CVD techniques using non-methyl containing group feed gases.
Hot filament CVD prior art is exemplified in the following: (a) Y. Hirose and Y. Terasawa, "Synthesis of Diamond Thin Films by Thermal CVD Using Organic Compounds", Japanese Journal of Applied Physics, Part 2, Volume 25, L519-21 (1986); (b) S. Matsumoto, Y. Sato, M. Kamo, N. Setaka, "Vapor Deposition of Diamond Particles from Methane", Japanese Journal of Applied Physics, Volume 21, No. 4, April 1982, pp. L183-L185; and (c) U.S. Pat. No. 4,707,384 and corresponding German Patent DE 3,522,583. All of these references rely on methyl group containing feed gases for the hot filament technique, although reference is made to using non-methyl gases when not working with hot filament techniques. The reason for the inability to explore non-methyl gases with the hot filament CVD technique is due to accepted attitudes (such as represented in U.S. Pat. Nos. 3,030,187 and 3,030,188) that non-methyl containing HC will not produce diamond growth.
The state of the art with respect to CVD techniques other than hot filament, such as microwave plasma and electron assisted heating, reference should be made to the following articles and patents: (a) K. Kobashi, K. Nishimura, Y. Kawate, T. Horiuchi, "Synthesis of Diamonds by Use of Microwave Plasma Chemical-Vapor Deposition: Morphology and Growth of Diamond Films", Physical Review, Volume 38, No. 6, Aug. 15, 1988 II, pages 4067-4083; and (b) U.S. Pat. Nos. 4,740,263, and 4,434,188.
The article to Kobashi et al is most emphatic in presenting a negative teaching with respect to hot filament CVD when using hydrocarbon feed gases which do not contain methyl group, such hydrocarbons being acetylene or ethylene . It is representative of the state of technical understanding that professes that it is not possible to use acetylene and ethylene with thermal filament CVD diamond methods (see the third paragraph, page 4071, of the article to Kobashi et al, Physics Review, 38 (1988)).
U.S. Pat. No. 4,740,263 is not helpful with respect to hot filament techniques since it utilizes acetylene only with respect to electron assisted CVD. In electron assisted CVD, extra energy must be supplied to cause diamond crystal nuclei to form and grow to a thin film with electron bombardment. The extra energy is supplied by DC voltage bias. A direct current voltage (for example, 150 volts with 20 mA/cm.sup.2 as described in example 1, and 1000 volts with 80 mA/cm.sup.2 as described in example 6) between the filament and the substrate (positive voltage) is required in order to emit and accelerate electrons and to cause bombardment on the substrate surface. Electron assisted CVD which causes ionization of molecules and radicals in gas Phase and on the substrate surface is different from hot filament CVD. Electron bombardment is not desirable because of a dislocation of the lattice structure. In U.S. Pat. No. 4,434,188, microwave plasma is used at unusually low temperatures; this leads to a much lower deposition rate and similar dislocation.